The overarching goal of the parent grant (R24 OD020347-01A1) is to establish standards for a nutritionally healthy, captive common marmoset (Callithrix jacchus). The common marmoset has become an increasingly popular nonhuman primate research model; however the full potential of this species as a research model is impeded by our lack of understanding of its nutritional requirements. The planned studies in the parent grant will identify the critical features of a standardized basic diet for captive common marmosets; determine links between diet, gut microbiome, and disease; and establish standards for healthy weights, body condition, and biomarkers of metabolic function. The Specific Aims of the parent grant are: (1) Conducting nutritional assessments regarding select macro- and micro-nutrient requirements for marmosets and their relations to biomarkers of gut function; (2) Validating markers of healthy weight and metabolic function in captive common marmosets; (3) Identifying links between nutrition and disease in captive common marmosets. An important refinement to the parent grant is the ability to use identical, modern equipment for the measurement of all components of the energy balance equation at both sites, greatly reducing the impact of inter-site variability and increasing the accuracy of identifying diet effects. To accomplish these refinements, we request support for the purchase of: (1) a point-of-care (POC) HbA1C immunoassay analyzer (DCA Advantage Analyzer, Siemens Healthcare Diagnostics, Malvern, PA) for each site to allow assessment of HbA1C more rapidly and importantly on a significantly smaller sample (1 l sample), a vital factor for this small-bodied species. This will facilitate the use of chronic glucose control as an important factor in understanding diet response and energy balance; (2) the same Echo MRI quantitative magnetic resonance unit (EchoMRI-Marmoset-700TM, EchoMRI LLC, Houston, TX) currently used by SNPRC for WNPRC. WNPRC currently analyzes body composition on sedated animals by dual-energy x-ray absorptiometry. The ability of the Echo MRI unit to rapidly, precisely and accurately measure lean and fat mass in awake marmosets is a clear advantage; (3) equipment (FoxBox respirometry system and Actiwatch Mini accelerometers) that will allow us to more completely characterize energy balance in the Aim 2 animals. These additional assessments will greatly improve our ability to evaluate metabolic function in captive common marmosets. The equipment that will be available through this revision application will greatly enhance the parent R24 by allowing more consistent and complete measurements of animals at the two study sites; enhancing our understanding of nutrition and metabolic function in common marmosets; and setting the standard for captive nutritional care in this species. Given the growing appreciation of the influence of metabolism on myriad other biological systems, the enhanced metabolic assessments proposed in this revision application will result in a better characterized, and thus more valuable, biomedical research model and support continued collaborative studies of the marmoset at SNPRC and WNPRC.